Tire tread compositions containing asymmetrically tin-coupled polybutadiene rubber and silica coupling agent

ABSTRACT

This invention relates to a tire tread compound that is easily processable which can be used to improve the treadwear, rolling resistance and traction characteristics of tires. The tire tread compounds of this invention are a blend of tin-coupled polybutadiene, high vinyl polybutadiene and natural rubber. This blend of low glass transition temperature rubber and high glass transition temperature rubber is surprisingly easy to process which makes the concept of this invention commercially feasible. Thus, the tire tread compounds of this invention can be utilized in making tires having greatly improved traction characteristics and treadwear without sacrificing rolling resistance. These improved properties may be due, in part, to better interaction and compatibility with carbon black and/or silica fillers. The polybutadiene in the blend can be asymmetrical tin-coupled to further improve the cold flow characteristics of the rubber blend. Asymmetrical tin coupling in general also leads to better processability and other beneficial properties. This invention more specifically discloses a tire tread rubber composition which is comprised of (1) from about 20 phr to about 60 phr of tin-coupled polybutadiene rubber, (2) from about 20 phr to about 60 phr of a rubber selected from the group consisting of natural rubber and synthetic polyisoprene and (3) from about 5 phr to about 40 phr of high vinyl polybutadiene rubber.

This is a Divisional of application Ser. No. 08/935,172, filed on Sep.22, 1997, presently pending.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to exhibit good tractioncharacteristics on both dry and wet surfaces. However, it hastraditionally been very difficult to improve the tractioncharacteristics of a tire without compromising its rolling resistanceand tread wear. Low rolling resistance is important because good fueleconomy is virtually always an important consideration. Good tread wearis also an important consideration because it is generally the mostimportant factor which determines the life of the tire.

The traction, tread wear and rolling resistance of a tire is dependentto a large extent on the dynamic viscoelastic properties of theelastomers utilized in making the tire tread. In order to reduce therolling resistance of a tire, rubbers having a high rebound havetraditionally been utilized in making the tire's tread. On the otherhand, in order to increase the wet skid resistance of a tire, rubberswhich undergo a large energy loss have generally been utilized in thetire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads. For instance,various mixtures of styrene-butadiene rubber and polybutadiene rubberare commonly used as a rubber material for automobile tire treads.However, such blends are not totally satisfactory for all purposes.

Rubbers having intermediate glass transition temperatures (-70° C. to-40° C.) compromise rolling resistance and treadwear withoutsignificantly increasing traction characteristics. For this reason,blends of rubbers having low glass transition temperatures and rubbershaving high glass transition temperatures are frequently utilized toattain improved traction characteristics without significantlycompromising rolling resistance or treadwear. However, such blends ofrubbers having low glass transition temperatures and rubbers having highglass transition temperatures exhibit poor processability. This majordisadvantage associated with such blends has greatly hampered theirutilization in making tire tread compounds.

Tin-coupled polymers are known to provide desirable properties, such asimproved treadwear and reduced rolling resistance, when used in tiretread rubbers. Such tin-coupled rubbery polymers are typically made bycoupling the rubbery polymer with a tin coupling agent at or near theend of the polymerization used in synthesizing the rubbery polymer. Inthe coupling process, live polymer chain ends react with the tincoupling agent thereby coupling the polymer. For instance, up to fourlive chain ends can react with tin tetrahalides, such as tintetrachloride, thereby coupling the polymer chains together.

SUMMARY OF THE INVENTION

This invention relates to a tire tread compound that is easilyprocessable which can be used to improve the treadwear, rollingresistance and traction characteristics of tires. The tire treadcompounds of this invention are a blend of tin-coupled polybutadiene,high vinyl polybutadiene and natural rubber. This blend of low glasstransition temperature rubber and high glass transition temperaturerubber is surprisingly easy to process which makes the concept of thisinvention commercially feasible. Thus, the tire tread compounds of thisinvention can be utilized in making tires having greatly improvedtraction characteristics and treadwear without sacrificing rollingresistance. These improved properties may be due, in part, to betterinteraction and compatibility with carbon black and/or silica fillers.The polybutadiene in the blend can be asymmetrically tin-coupled tofurther improve the cold flow characteristics of the rubber blend.Asymmetrical tin coupling in general also leads to better processabilityand other beneficial properties.

This invention more specifically discloses a tire tread rubbercomposition which is comprised of (1) from about 20 phr to about 60 phrof tin-coupled polybutadiene rubber, (2) from about 20 phr to about 60phr of a rubber selected from the group consisting of natural rubber andsynthetic polyisoprene and (3) from about 5 phr to about 40 phr of highvinyl polybutadiene rubber.

It is normally preferred for the tin-coupled polybutadiene rubber to beasymmetrically tin-coupled. In such cases, the stability of blendscontaining asymmetrical tin-coupled polybutadiene rubber can be improvedby adding a tertiary chelating amine thereto subsequent to the time atwhich the tin-coupled rubbery polymer is coupled.N,N,N',N'-tetramethylethylenediamine (TMEDA) is a representative exampleof a tertiary chelating amine which is preferred for utilization instabilizing the polymer blends of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The tire tread rubber compositions of this invention are comprised of(1) from about 20 phr to about 60 phr of tin-coupled polybutadienerubber, (2) from about 20 phr to about 60 phr of a rubber selected fromthe group consisting of natural rubber and synthetic polyisoprene and(3) from about 5 phr to about 40 phr of high vinyl polybutadiene rubber.These tire tread rubbers will typically contain from about 25 phr toabout 55 phr of the tin-coupled polybutadiene rubber, from about 25 phrto about 55 phr of the rubber selected from the group consisting ofnatural rubber and synthetic polyisoprene and from about 10 phr to about30 phr of the high vinyl polybutadiene rubber. It is normally preferredfor the tire tread rubber to contain from about 30 phr to about 50 phrof the tin-coupled polybutadiene rubber, from about 30 phr to about 50phr of the rubber selected from the group consisting of natural rubberand synthetic polyisoprene and from about 15 phr to about 25 phr of thehigh vinyl polybutadiene rubber.

The high vinyl polybutadiene rubber employed in the blends of thisinvention will normally have a glass transition temperature which iswithin the range of about -40° C. to +40° C. and a Mooney ML 1+4viscosity which is within the range of about 30 to about 100. The highvinyl polybutadiene rubber employed in the blends of this invention willpreferably have a glass transition temperature which is within the rangeof about -35° C. to 0° C. and a Mooney ML 1+4 viscosity which is withinthe range of about 40 to about 90. The high vinyl polybutadiene rubberemployed in the blends of this invention will preferably have a glasstransition temperature which is within the range of about -30° C. to-20° C. and a Mooney ML 1+4 viscosity which is within the range of about60 to about 80.

The tin-coupled polybutadiene will typically have a Mooney ML 1+4viscosity which is within the range of about 5 to about 40 beforecoupling and a Mooney ML 1+4 viscosity of about 60 to about 120 aftercoupling. The tin-coupled polybutadiene will preferably have a Mooney ML1+4 viscosity which is within the range of about 5 to about 35 beforecoupling and a Mooney ML 1+4 viscosity of about 75 to about 110 aftercoupling. The tin-coupled polybutadiene will most preferably have aMooney ML 1+4 viscosity which is within the range of about 10 to about30 before coupling and a Mooney ML 1+4 viscosity of about 80 to about100 after coupling.

The tin-coupled polybutadiene will typically be prepared by reacting"living" polybutadiene having lithium end groups with a tin halide, suchas tin tetrachloride. This coupling step will normally be carried out asa batch process. However, it is generally preferred to tin-couple thepolybutadiene in a continuous process which results in the formation ofasymmetrically tin-coupled polybutadiene rubber. A technique forproducing asymmetrically tin-coupled polybutadiene rubber is disclosedin U.S. Provisional patent application Ser. No. 60/037,929, filed onFeb. 14, 1997. The teachings of U.S. Provisional patent application Ser.No. 60/037,929 are hereby incorporated herein by reference in theirentirety.

The tin coupling agent employed in making asymmetrically tin-coupledpolybutadiene rubber will normally be a tin tetrahalide, such as tintetrachloride, tin tetrabromide, tin tetrafluoride or tin tetraiodide.However, tin trihalides can also optionally be used. In cases where tintrihalides are utilized, a coupled polymer having a maximum of threearms results. To induce a higher level of branching, tin tetrahalidesare normally preferred. As a general rule, tin tetrachloride is mostpreferred.

Broadly, and exemplary, a range of about 0.01 to 4.5 milliequivalents oftin coupling agent is employed per 100 grams of the rubbery polymer. Itis normally preferred to utilize about 0.01 to about 1.5milliequivalents of the tin coupling agent per 100 grams of polymer toobtain the desired Mooney viscosity. The larger quantities tend toresult in production of polymers containing terminally reactive groupsor insufficient coupling. One equivalent of tin coupling agent perequivalent of lithium is considered an optimum amount for maximumbranching. For instance, if a tin tetrahalide is used as the couplingagent, one mole of the tin tetrahalide would be utilized per four molesof live lithium ends. In cases where a tin trihalide is used as thecoupling agent, one mole of the tin trihalide will optimally be utilizedfor every three moles of live lithium ends. The tin coupling agent canbe added in a hydrocarbon solution, e.g., in cyclohexane, to thepolymerization admixture in the reactor with suitable mixing fordistribution and reaction.

After the tin coupling has been completed, a tertiary chelating alkyl1,2-ethylene diamine can optionally be added to the polymer cement tostabilize the tin-coupled rubbery polymer. This technique forstabilization of the tin-coupled rubber is more fully described in U.S.patent application Ser. No. 08/791,929, filed on Jan. 31, 1997 U.S. Pat.No. 5,739,182. The teachings of U.S. patent application Ser. No.08/791,929 are incorporated herein by reference in their entirety. Thetertiary chelating amines which can be used for stabilization arenormally chelating alkyl diamines of the structural formula: ##STR1##wherein n represents an integer from 1 to about 6, wherein A representsan alkane group containing from 1 to about 6 carbon atoms and whereinR¹, R², R³ and R⁴ can be the same or different and represent alkanegroups containing from 1 to about 6 carbon atoms. The alkane group A isthe formula .paren open-st.CH₂ .paren close-st._(m) wherein m is aninteger from 1 to about 6. The alkane group will typically contain from1 to 4 carbon atoms (m will be 1 to 4) and will preferably contain 2carbon atoms. In most cases, n will be an integer from 1 to about 3 withit being preferred for n to be 1. It is preferred for R¹, R², R³ and R⁴to represent alkane groups which contain from 1 to 3 carbon atoms. Inmost cases, R¹, R², R³ and R⁴ will represent methyl groups.

A sufficient amount of the chelating amine should be added to complexwith any residual tin coupling agent remaining after completion of thecoupling reaction. In most cases, from about 0.01 phr (parts by weightper 100 parts by weight of dry rubber) to about 2 phr of the chelatingalkyl 1,2-ethylene diamine will be added to the polymer cement tostabilize the rubbery polymer. Typically, from about 0.05 phr to about 1phr of the chelating alkyl 1,2-ethylene diamine will be added. Moretypically, from about 0.1 phr to about 0.6 phr of the chelating alkyl1,2-ethylene diamine will be added to the polymer cement to stabilizethe rubbery polymer.

After the polymerization, asymmetrical tin coupling and optionally thestabilization step, has been completed, the tin-coupled rubbery polymercan be recovered from the organic solvent utilized in the solutionpolymerization. The tin-coupled rubbery polymer can be recovered fromthe organic solvent and residue by means such as decantation,filtration, centrification and the like. It is often desirable toprecipitate the tin-coupled rubbery polymer from the organic solvent bythe addition of lower alcohols containing from about 1 to about 4 carbonatoms to the polymer solution. Suitable lower alcohols for precipitationof the rubber from the polymer cement include methanol, ethanol,isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the tin-coupled rubberypolymer from the polymer cement also "kills" any remaining livingpolymer by inactivating lithium end groups. After the tin-coupledrubbery polymer is recovered from the solution, steam-stripping can beemployed to reduce the level of volatile organic compounds in thetin-coupled rubbery polymer.

The asymmetrical tin-coupled polybutadiene rubber that can be employedin the blends of this invention are comprised of a tin atom having atleast three polybutadiene arms covalently bonded thereto. At least oneof the polybutadiene arms bonded to the tin atom has a number averagemolecular weight of less than about 40,000 and at least one of thepolybutadiene arms bonded to the tin atom has a number average molecularweight of at least about 80,000. The ratio of the weight averagemolecular weight to the number average molecular weight of theasymmetrical tin-coupled polybutadiene rubber will also normally bewithin the range of about 2 to about 2.5.

The asymmetrical tin-coupled polybutadiene rubber that can be utilizedin the blends of this invention is typically of the structural formula:##STR2## wherein R¹, R², R³ and R⁴ can be the same or different and areselected from the group consisting of alkyl groups and polybutadienearms (polybutadiene rubber chains), with the proviso that at least threemembers selected from the group consisting of R¹, R², R³ and R⁴ arepolybutadiene arms, with the proviso that at least one member selectedfrom the group consisting of R¹, R², R³ and R⁴ is a low molecular weightpolybutadiene arm having a number average molecular weight of less thanabout 40,000, with the proviso that at least one member selected fromthe group consisting of R¹, R², R³ and R⁴ is a high molecular weightpolybutadiene arm having a number average molecular weight of greaterthan about 80,000, and with the proviso that the ratio of the weightaverage molecular weight to the number average molecular weight of theasymmetrical tin-coupled polybutadiene rubber is within the range ofabout 2 to about 2.5. It should be noted that R¹, R², R³ and R⁴ can bealkyl groups because it is possible for the tin halide coupling agent toreact directly with alkyl lithium compounds which are used as thepolymerization initiator.

In most cases, four polybutadiene arms will be covalently bonded to thetin atom in the asymmetrical tin-coupled polybutadiene rubber. In suchcases, R¹, R², R³ and R⁴ will all be polybutadiene arms. Theasymmetrical tin-coupled polybutadiene rubber will often contain apolybutadiene arm of intermediate molecular weight as well as the lowmolecular weight arm and the high molecular weight arm. Suchintermediate molecular weight arms will have a molecular weight which iswithin the range of about 45,000 to about 75,000. It is normallypreferred for the low molecular polybutadiene arm to have a molecularweight of less than about 30,000, with it being most preferred for thelow molecular weight arm to have a molecular weight of less than about25,000. It is normally preferred for the high molecular polybutadienearm to have a molecular weight of greater than about 90,000, with itbeing most preferred for the high molecular weight arm to have amolecular weight of greater than about 100,000.

The tire tread rubber compositions of this invention can be compoundedutilizing conventional ingredients and standard techniques. Forinstance, these tire tread rubber blends will typically be mixed withcarbon black and/or silica, sulfur, fillers, accelerators, oils, waxes,scorch inhibiting agents and processing aids. In most cases, the rubberblend will be compounded with sulfur and/or a sulfur-containingcompound, at least one filler, at least one accelerator, at least oneantidegradant, at least one processing oil, zinc oxide, optionally atackifier resin, optionally a reinforcing resin, optionally one or morefatty acids, optionally a peptizer and optionally one or more scorchinhibiting agents. Such blends will normally contain from about 0.5 to 5phr (parts per hundred parts of rubber by weight) of sulfur and/or asulfur-containing compound with 1 phr to 2.5 phr being preferred. It maybe desirable to utilize insoluble sulfur in cases where bloom is aproblem.

At least some silica will be utilized in the blend as a filler. Thefiller can, of course, be comprised totally of silica. However, in somecases, it will be beneficial to utilize a combination of silica andcarbon black as the filler. Clays and/or talc can be included in thefiller to reduce cost. The blend will also normally include from 0.1 to2.5 phr of at least one accelerator with 0.2 to 1.5 phr being preferred.Antidegradants, such as antioxidants and antiozonants, will generally beincluded in the tread compound blend in amounts ranging from 0.25 to 10phr with amounts in the range of 1 to 5 phr being preferred. Processingoils will generally be included in the blend in amounts ranging from 2to 100 phr with amounts ranging from 5 to 50 phr being preferred. Thepolybutadiene blends of this invention will also normally contain from0.5 to 10 phr of zinc oxide with 1 to 5 phr being preferred. Theseblends can optionally contain from 0 to 10 phr of tackifier resins, 0 to10 phr of reinforcing resins, 1 to 10 phr of fatty acids, 0 to 2.5 phrof peptizers and 0 to 1 phr of scorch inhibiting agents.

To fully realize the total advantages of the blends of this invention,silica will normally be included in the tread rubber formulation. Theprocessing of the rubber blend is normally conducted in the presence ofa sulfur containing organosilicon compound as a silica coupling agent torealize maximum benefits. Examples of suitable sulfur-containingorganosilicon compounds are of the formula:

    Z-Alk-S.sub.n -Alk-Z                                       (I)

in which Z is selected from the group consisting of ##STR3## where R¹ isan alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R²is alkoxy of 1 to 8 carbon atoms or cycloalkoxy of 5 to 8 carbon atoms;and wherein Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and nis an integer of 2 to 8.

Specific examples of sulfur-containing organosilicon compounds which maybe used in accordance with the present invention include:3,3'-bis(trimethoxysilylpronyl) disulfide,3,3'-bis(triethoxysilylpropyl) tetrasulfide,3,3'-bis(triethoxysilylpropyl) octasulfide,3,3'-bis(trimethoxysilylpropyl) tetrasulfide,2,2'-bis(triethoxysilylethyl) tetrasulfide,3,3'-bis(trimethoxysilylpropyl) trisulfide,3,3'-bis(triethoxysilylpropyl) trisulfide,3,3'-bis(tributoxysilylpropyl) disulfide,3,3'-bis(trimethoxysilylpropyl) hexasulfide,3,3'-bis(trimethoxysilylpropyl) octasulfide,3,3'-bis(trioctoxysilylpropyl) tetrasulfide,3,3'-bis(trihexoxysilylpropyl) disulfide,3,3'-bis(tri-2"-ethylhexoxysilylpropyl) trisulfide,3,3'-bis(triisooctoxysilylpropyl) tetrasulfide,3,3'-bis(tri-t-butoxysilylpropyl) disulfide, 2,2'-bis(methoxy diethoxysilyl ethyl) tetrasulfide, 2,2'-bis(tripropoxysilylethyl) pentasulfide,3,3'-bis(tricyclonexoxysilylpropyl) tetrasulfide,3,3'-bis(tricyclopentoxysilylpropyl) trisulfide,2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide,bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl3'-diethoxybutoxy-silylpropyltetrasulfide, 2,2'-bis(dimethylmethoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide,3,3'-bis (di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenylmethyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenylisopropoxysilylpropyl) tetrasulfide, 3,3'-bis(diphenylcyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyldimethoxysilylethyl) trisulfide, 2,2'-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec.butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl)disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide,3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenylethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide,4,4'-bis(trimethoxysilylbutyl) tetrasulfide,6,6'-bis(triethoxysilylhexyl) tetrasulfide,12,12'-bis(triisopropoxysilyl dodecyl) disulfide,18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5'-bis(dimethoxymethylsilylpentyl) trisulfide,3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide,3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur-containing organosilicon compounds are the3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The mostpreferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide.Therefore, as to Formula I, preferably Z is ##STR4## where R² is analkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularlypreferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3carbon atoms being particularly preferred; and n is an integer of from 3to 5 with 4 being particularly preferred.

The amount of the sulfur-containing organosilicon compound of Formula Iin a rubber composition will vary, depending on the level of silica thatis used. Generally speaking, the amount of the compound of Formula Iwill range from about 0.01 to about 1.0 parts by weight per part byweight of the silica. Preferably, the amount will range from about 0.02to about 0.4 parts by weight per part by weight of the silica. Morepreferably, the amount of the compound of formula I will range fromabout 0.05 to about 0.25 parts by weight per part by weight of thesilica.

In addition to the sulfur-containing organosilicon, the rubbercomposition should contain a sufficient amount of silica, and carbonblack, if used, to contribute a reasonably high modulus and highresistance to tear. The silica filler may be added in amounts rangingfrom about 10 phr to about 250 phr. Preferably, the silica is present inan amount ranging from about 15 phr to about 80 phr. If carbon black isalso present, the amount of carbon black, if used, may vary. Generallyspeaking, the amount of carbon black will vary from about 5 phr to about80 phr. Preferably, the amount of carbon black will range from about 10phr to about 40 phr. It is to be appreciated that the silica coupler maybe used in conjunction with a carbon black, namely pre-mixed with acarbon black prior to addition to the rubber composition, and suchcarbon black is to be included in the aforesaid amount of carbon blackfor the rubber composition formulation. In any case, the total quantityof silica and carbon black will be at least about 30 phr. The combinedweight of the silica and carbon black, as hereinbefore referenced, maybe as low as about 30 phr, but is preferably from about 45 to about 130phr.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate;e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300. The silica might beexpected to have an average ultimate particle size, for example, in therange of 0.01 to 0.05 micron as determined by the electron microscope,although the silica particles may be even smaller, or possibly larger,in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3.

Tire tread formulations which include silica and an organosiliconcompound will typically be mixed utilizing a thermomechanical mixingtechnique. The mixing of the tire tread rubber formulation can beaccomplished by methods known to those having skill in the rubber mixingart. For example, the ingredients are typically mixed in at least twostages, namely at least one non-productive stage followed by aproductive mix stage. The final curatives including sulfur-vulcanizingagents are typically mixed in the final stage which is conventionallycalled the "productive" mix stage in which the mixing typically occursat a temperature, or ultimate temperature, lower than the mixtemperature(s) than the preceding non-productive mix stage(s). Therubber, silica and sulfur-containing organosilicon, and carbon black, ifused, are mixed in one or more non-productive mix stages. The terms"non-productive", and "productive" mix stages are well known to thosehaving skill in the rubber mixing art. The sulfur-vulcanizable rubbercomposition containing the sulfur-containing organosilicon compound,vulcanizable rubber and generally at least part of the silica should besubjected to a thermomechanical mixing step. The thermomechanical mixingstep generally comprises a mechanical working in a mixer or extruder fora period of time suitable in order to produce a rubber temperaturebetween 140° C. and 190° C. The appropriate duration of thethermomechanical working varies as a function of the operatingconditions and the volume and nature of the components. For example, thethermomechanical working may be for a duration of time which is withinthe range of about 2 minutes to about 20 minutes. It will normally bepreferred for the rubber to reach a temperature which is within therange of about 145° C. to about 180° C. and to be maintained at saidtemperature for a period of time which is within the range of about 4minutes to about 12 minutes. It will normally be more preferred for therubber to reach a temperature which is within the range of about 155° C.to about 170° C. and to be maintained at said temperature for a periodof time which is within the range of about 5 minutes to about 10minutes.

The tire tread compounds of this invention can be used in tire treads inconjunction with ordinary tire manufacturing techniques. Tires are builtutilizing standard procedures with the polybutadiene rubber blend simplybeing substituted for the rubber compounds typically used as the treadrubber. After the tire has been built with the polybutadienerubber-containing blend, it can be vulcanized using a normal tire curecycle. Tires made in accordance with this invention can be cured over awide temperature range. However, it is generally preferred for the tiresof this invention to be cured at a temperature ranging from about 132°C. (270° F.) to about 166° C. (330° F.). It is more typical for thetires of this invention to be cured at a temperature ranging from about143° C. (290° F.) to about 154° C. (310° F.). It is generally preferredfor the cure cycle used to vulcanize the tires of this invention to havea duration of about 10 to about 20 minutes with a cure cycle of about 12to about 18 minutes being most preferred.

By utilizing the rubber blends of this invention in tire treadcompounds, traction characteristics can be improved without compromisingtread wear or rolling resistance. Since the polybutadiene rubber blendsof this invention do not contain styrene, the cost of raw materials canalso be reduced. This is because styrene and other vinyl aromaticmonomers are expensive relative to the cost of 1,3-butadiene.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

EXMAPLE 1

In this experiment, a tin-coupled polybutadiene rubber was prepared in a10-gallon (38 liter) batch reactor at a temperature of 70° C. In theprocedure used, 22,400 grams of a silica/molecular sieve/aluminum driedpremix containing 17.5 weight percent of 1,3-butadiene in hexanes wascharged into the 10-gallon reactor. After the amount of impurity in thepremix was determined, 28.8 ml of 1.6 M solution of n-butyl lithium (inhexane) was added to the reactor. The target Mn (number averagedmolecular weight) was 100,000. The polymerization was allowed to proceedat 70° C. for two hours. An analysis of the residual monomer indicatedthat all monomers were totally consumed. After a small aliquot ofpolymer cement was removed from the reactor (for analysis), 9.2 ml of a0.65 M solution of tin tetrachloride (in hexane) was added to thereactor and the coupling reaction was carried out at the sametemperature for 30 minutes. At this time, 1.5 phr (parts per 100 partsby weight of rubber) of antioxidant was added to the reactor toshortstop the polymerization and to stabilize the polymer.

After the hexane solvent was evaporated, the resulting tin-coupledpolybutadiene was dried in a vacuum oven at 50° C. The tin-coupledpolybutadiene was determined to have a glass transition temperature (Tg)at -95° C. It was also determined to have a microstructure whichcontained 8 percent 1,2-polybutadiene units and 92 percent1,4-polybutadiene units. The Mooney viscosity (ML 1+4@100° C.) of thetin-coupled polybutadiene made was determined to be 110. The MooneyViscosity of the base polybutadiene rubber was also determined to be 11.

EXAMPLE 2

In this experiment, asymmetrically tin-coupled polybutadiene wassynthesized in a three-reactor (2 gallons each) continuous system at 90°C. A premix containing 15 percent 1,3-butadiene in hexane was chargedinto the first reactor continuously at a rate of 117 grams/minute.Polymerization was initiated by adding a 0.128 M solution of n-butyllithium into the first reactor at a rate of 0.82 grams/minute. Most ofmonomers were exhausted at the end of second reactor and thepolymerization medium containing live lithium ends was continuouslypushed into the third reactor where the coupling agent, tintetrachloride (0.025 M solution in hexane), was added at a rate of 1.16grams/minutes. The residence time for all three reactors was set at 1.5hours to achieve complete monomer conversion in the second reactor andcomplete coupling in the third reactor. The polymerization medium wasthen continuously pushed over to a holding tank containing a shortstopand an antioxidant. The resulting polymer cement was then steam-strippedand the asymmetrical tin-coupled polybutadiene recovered was dried in anoven at 60° C. The polymer was determined to have a glass transitiontemperature at -95° C. and have a Mooney ML 1+4@100° C. viscosity of 94.It was also determined to have a microstructure which contained 8percent 1,2-polybutadiene units and 92 percent 1,4-polybutadiene units.The precursor of this polymer (i.e., base polymer prior to coupling) wasalso determined to have an ML 1+4@100° C. of 20.

EXAMPLES 3-4 AND COMPARATIVE EXAMPLES 5-8

In this series of experiments, various rubber blends were prepared andevaluated as tire tread rubber compositions. These blends were preparedby a three-step mixing process. In the first step, non-productive blendswere made by mixing the rubbers shown in Table I with 7.0 parts ofprocessing aids, 3 parts of zinc oxide, 2 parts of stearic acid and 0.15parts of 2,2'-dibenzamidodiphenyl disulfide. This first non-productivemixing step was carried out over a period of about 4 minutes whichresulted in a temperature of about 160° C. being attained.

In the procedure used, 12 parts of fine particle-size hydrated silica,2.25 parts of a 50 percent/50 percent blend of silica and carbon black(X50S from DeGussa GmbH) and 3 parts of a naphthenic/paraffinic processoil were added to the blend in a second non-productive mixing step. Thissecond non-productive mixing step was carried out over a period of about3 minutes to a temperature of about 150° C.

A productive compound was then made by mixing 0.66 parts ofdiaryl-p-phenylenediamine, 1.12 parts of N-tert-butyl-2-benzothiazole,0.14 parts of tetramethylthiuram disulfide and 1.5 parts of rubbermakers sulfur into the blend. This productive mixing step was carriedout over a period of about 2.5 minutes to a drop temperature of about120° C. Then, the tire tread rubber compounds were cured and evaluated.The results of this evaluation are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Example          3         4       5                                          ______________________________________                                        Natural Rubber.sup.1                                                                           40        49      40                                           Isoprene-Butadiene Rubber.sup.2          45                                   3,4-Polyisoprene.sup.3                    6                                   High Vinyl Polybutadiene.sup.4    20            20                            Polybutadiene Rubber.sup.5        40                                          Tin-Coupled Polybutadiene.sup.6                 40                            Carbon Black.sup.7                38            38                            Silica.sup.8                      12            12                            Rheometer 150° C.                                                      Min torque                       11     11     9.6                            Max torque                     40.8   41.3     42                             delta torque                   29.8   30.3   32.4                             T25                              6    6.25   6.75                             T90                            9.75   9.75   10.5                             ATS 18 @ 150                                                                  100% Modulus, MPa              2.39   2.35   2.31                             300% Modulus, MPa             11.07  11.09  11.83                             Brk Str, MPa                  16.61  19.27  16.94                             EL-Brk, %                      441    489    418                              Hardness, RT                  61.6   61.6   60.9                              Hardness, 100° C.        58.8   58.4   58.2                            Rebound RT %                  56.8   56.1   61                                Rebound, 100° C.         68.9   71.4   71.9                            DIN                            73     96     72                               Tan Delta                                                                     -40               0.53   0.43   0.51                                          -30               0.32   0.26   0.30                                          -20               0.22   0.18   0.20                                          -10               0.15   0.14   0.13                                           0                  0.13   0.13   0.12                                      ______________________________________                                         .sup.1 TSR20                                                                  .sup.2 30% Isoprene/70% Butadiene, Tg = -82° C., Mooney ML/4 @         100° C. = 85                                                           .sup.3 365% 3,4structure, Mooney ML/4 @ 100° C. = 70                   .sup.4 80% Vinyl; 82 Mooney, Tg2                                              .sup.5 Budene 1209 from -28° C.                                        .sup.6 Polymer of Example 1                                                   .sup.7 ASTM N299                                                              .sup.8 HiSil 210 from PPG                                                

It is well known that Sn-coupled polymers provide improvements inprocessing over their linear counterparts. You see an example of thiswhen comparing the Rheometer minimum torque values of Example 5 versusExample 3. The compound of Example 5 contains tin-coupled polybutadieneand has the lower minimum torque. The rebound values of the compoundcontaining the tin-coupled polybutadiene (Example 5) are higher than forthe control Example 3, suggesting better rolling resistance (RR) for thecompound containing the tin-coupled polybutadiene.

Example 4 is an example of current passenger tread technology. Thistread contains a blend of three polymers: (1) NR for processing andtraction, (2) IBR for processing, treadwear and RR and (3) 3,4polyisoprene for traction. The compound of this invention (Example 5)also uses a blend of three polymers, including NR, high vinylpolybutadiene for traction and Sn-coupled polybutadiene for processing,RR and treadwear.

The lab data (Table I) clearly shows the superiority of the new polymersystem over that of the current polymer system. The new polymer systemprovides improved treadwear (DIN abrasion resistance improved 25percent), along with reduced rolling resistance (RT rebound valuesincreased 9 percent). The wet traction (measured by the tan delta valuesfrom -40° C. through 0° C.) should be equal between the two compounds,as can be seen from the tan delta values reported in Table I. It shouldbe noted that higher tan delta values in the range of -40° C. to 0° C.are predictive of better wet reaction characteristics in tires.

In summary, the new polymer system (Example 5) improves the tradeoffbetween PR, treadwear and wet traction versus current technology(Example 4) and the control polymer system (Example 3).

Variations in the present invention are possible in light of thedescription of it provided herein. While certain representativeembodiments and details have been shown for the purpose of illustratingthe subject invention, it will be apparent to those skilled in this artthat various changes and modifications can be made therein withoutdeparting from the scope of the subject invention. It is, therefore, tobe understood that changes can be made in the particular embodimentsdescribed which will be within the full intended scope of the inventionas defined by the following appended claims.

What is claimed is:
 1. A tire tread rubber composition which iscomprised of (1) from about 20 phr to about 60 phr of an asymmetricallytin-coupled polybutadiene rubber, (2) from about 20 phr to about 60 phrof a rubber selected from the group consisting of natural rubber andsynthetic polyisoprene, (3) from about 5 phr to about 40 phr of highvinyl polybutadiene rubber having a vinyl content of at least about 80percent, wherein the high vinyl polybutadiene rubber is a high vinylpolybutadiene rubber other than said asymmetrically tin-coupledpolybutadiene rubber, (4) from about 10 phr to about 250 phr of silica,(5) from about 5 phr to about 80 phr of carbon black and (6) a silicacoupling agent.
 2. A tire tread rubber composition as specified in claim1 wherein the silica and the silica coupling agent are mixed into thecomposition utilizing a thermomechanical mixing technique wherein therubber composition is maintained at a temperature which is within therange of about 145° C. to about 180° C. for about 4 minutes to about 12minutes.
 3. A tire tread rubber composition as specified in claim 2wherein the total quantity of the silica and the carbon black is atleast about 30 phr.
 4. A tire tread rubber composition as specified inclaim 3 wherein said tin-coupled polybutadiene rubber has a Mooney ML1+4 viscosity at 100° C. which is within the range of about 60 to about120.
 5. A tire tread rubber composition as specified in claim 3 whereinsaid asymmetrically tin-coupled polybutadiene rubber is tin-coupled in acontinuous process.
 6. A tire tread rubber composition as specified inclaim 3 wherein said high vinyl polybutadiene rubber has a Mooney ML 1+4viscosity at 100° C. which is within the range of about 30 to about 100.7. A tire tread rubber composition as specified in claim 4 wherein saidmember selected from the group consisting of natural rubber andsynthetic polyisoprene is natural rubber.
 8. A tire tread rubbercomposition as specified in claim 4 wherein said member selected fromthe group consisting of natural rubber and synthetic polyisoprene issynthetic polyisoprene rubber.
 9. A tire tread rubber composition asspecified in claim 6 wherein the tire tread rubber composition containsfrom about 25 phr to about 55 phr of the asymmetrically tin-coupledpolybutadiene rubber, from about 25 phr to about 55 phr of the rubberselected from the group consisting of natural rubber and syntheticpolyisoprene, from about 10 phr to about 30 phr of the high vinylpolybutadiene rubber, from about 15 phr to about 80 phr of the silicaand from about 10 phr to about 40 phr of the carbon black.
 10. A tiretread rubber composition as specified in claim 9 wherein the totalquantity of the silica and the carbon black present is within the rangeof about 45 phr to about 130 phr.
 11. A tire tread rubber composition asspecified in claim 10 wherein the tire tread rubber composition containsfrom about 30 phr to about 50 phr of the asymmetrically tin-coupledpolybutadiene rubber, from about 30 phr to about 50 phr of the rubberselected from the group consisting of natural rubber and syntheticpolyisoprene and from about 15 phr to about 25 phr of the high vinylpolybutadiene rubber.
 12. A tire tread rubber composition as specifiedin claim 11 wherein the silica has a Bet surface area which is withinthe range of about 40 to about 600 m² /g.
 13. A tire tread rubbercomposition as specified in claim 12 wherein the silica has an averageparticle size which is within the range of 0.01 microns to 0.05 micronsas determined with an electron microscope.
 14. A tire tread rubbercomposition as specified in claim 13 wherein the high vinylpolybutadiene rubber has a Mooney ML 1+4 viscosity at 100° C. which iswithin the range of about 60 to about
 80. 15. A tire tread rubbercomposition as specified in claim 14 wherein said asymmetricallytin-coupled polybutadiene rubber has a Mooney ML 1+4 viscosity at 100°C. which is within the range of about 75 to about
 110. 16. A tire treadrubber composition as specified in claim 14 wherein said asymmetricallytin-coupled polybutadiene rubber has a Mooney ML 1+4 viscosity at 100°C. which is within the range of about 80 to about
 100. 17. A tire treadrubber composition as specified in claim 15 wherein said asymmetricallytin-coupled polybutadiene is of the structural formula: ##STR5## whereinR¹, R², R³ and R⁴ can be the same or different and are selected from thegroup consisting of alkyl groups and polybutadiene arms, with theproviso that at least three members selected from the group consistingof R¹, R², R³ and R⁴ are polybutadiene arms, with the proviso that atleast one member selected from the group consisting of R¹, R², R³ and R⁴is a low molecular weight polybutadiene arm having a number averagemolecular weight of less than about 40,000, with the proviso that atleast one member selected from the group consisting of R¹, R², R³ and R⁴is a high molecular weight polybutadiene arm having a number averagemolecular weight of greater than about 80,000 and with the proviso thatthe ratio of the weight average molecular weight to the number averagemolecular weight of the asymmetrical tin-coupled polybutadiene rubber iswithin the range of about 2 to about 2.5.
 18. A tire tread rubbercomposition as specified in claim 17 wherein R¹, R², R³ and R⁴ are allpolybutadiene arms.
 19. A tire tread rubber composition as specified inclaim 17 wherein the low molecular weight polybutadiene arm has a numberaverage molecular weight of less than about 30,000.
 20. A tire treadrubber composition as specified in claim 19 wherein the high molecularweight polybutadiene arm has a number average molecular weight ofgreater than about 90,000.
 21. A tire tread rubber composition asspecified in claim 17 wherein the silica and the silica coupling agentare mixed into the composition in the thermomechanical mixing techniquewhile the rubber composition is maintained at a temperature which iswithin the range of about 155° C. to about 170° C. for about 5 minutesto about 10 minutes.
 22. A tire tread rubber composition as specified inclaim 1 wherein the silica coupling agent is present at a level which iswithin the range of about 0.01 to about 1.0 parts by weight per part byweight of the silica.
 23. A tire tread rubber composition as specifiedin claim 9 wherein the silica coupling agent is present at a level whichis within the range of about 0.02 to about 0.4 parts by weight per partby weight of the silica.
 24. A tire tread rubber composition asspecified in claim 17 wherein the silica coupling agent is present at alevel which is within the range of about 0.05 to about 0.25 parts byweight per part by weight of the silica.